class_docstr: Timetable that schedules the execution once as soon as possible.
  
  This corresponds to ``schedule="@once"``.
code: def summary(self) -> str:
      return "@once"
  
imports: from __future__ import annotations
  import operator
  from typing import TYPE_CHECKING, Any, Collection
  from pendulum import DateTime
  from airflow.timetables.base import DagRunInfo, DataInterval, Timetable
label_desc: Use image node_id 1 for calling the OnceTimetable obj's underlying member method code with example usage: obj.summary() and returns: str